96 research outputs found
On the origin of 140 GHz emission from the 4 July 2012 solar flare
The sub-THz event observed on the 4 July 2012 with the Bauman Moscow State
Technical University Radio Telescope RT-7.5 at 93 and 140~GHz as well as
Kislovodsk and Mets\"ahovi radio telescopes, Radio Solar Telescope Network
(RSTN), GOES, RHESSI, and SDO orbital stations is analyzed. The spectral flux
between 93 and 140 GHz has been observed increasing with frequency. On the
basis of the SDO/AIA data the differential emission measure has been
calculated. It is shown that the thermal coronal plasma with the temperature
above 0.5~MK cannot be responsible for the observed sub-THz flare emission. The
non-thermal gyrosynchrotron mechanism can be responsible for the microwave
emission near ~GHz but the observed millimeter spectral characteristics are
likely to be produced by the thermal bremsstrahlung emission from plasma with a
temperature of about 0.1~MK.Comment: 18 pages, 6 figure
Synchrotron emission from the T Tauri binary system V773 Tau A
The pre-main sequence binary system V773 Tau A shows remarkable flaring
activity around periastron passage. Here, we present the observation of such a
flare at a wavelength of 3 mm (90 GHz) performed with the Plateau de Bure
Interferometer. We examine different possible causes for the energy losses
responsible for the e-folding time of 2.3 hours of that flare. We exclude
synchrotron, collisional, and inverse Compton losses because they are not
consistent with observational constraints, and we propose that the fading of
the emission is due to the leakage of electrons themselves at each reflection
between the two mirror points of the magnetic structure partially trapping
them. The magnetic structure compatible with both our leakage model and
previous observations is that of a helmet streamer that, as in the solar case,
can occur at the top of the X-ray-emitting, stellar-sized coronal loops of one
of the stars. The streamer may extend up to 20 R and interact with the corona
of the other star at periastron passage, causing recurring flares. The inferred
magnetic field strength at the two mirror points of the helmet streamer is in
the range 0.12 - 125 G, and the corresponding Lorentz factor, gamma, of the
partially trapped electrons is in the range 20 < gamma < 632. We therefore rule
out that the emission could be of gyro-synchrotron nature: the derived high
Lorentz factor proves that the nature of the emission at 90 GHz from this
pre-main binary system is synchrotron radiation.
Based on observations carried out with the IRAM Plateau de Bure
Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN
(Spain).Comment: 8 pages, 5 figures, A&A in pres
Sub-terahertz, microwaves and high energy emissions during the December 6, 2006 flare, at 18:40 UT
The presence of a solar burst spectral component with flux density increasing
with frequency in the sub-terahertz range, spectrally separated from the
well-known microwave spectral component, bring new possibilities to explore the
flaring physical processes, both observational and theoretical. The solar event
of 6 December 2006, starting at about 18:30 UT, exhibited a particularly
well-defined double spectral structure, with the sub-THz spectral component
detected at 212 and 405 GHz by SST and microwaves (1-18 GHz) observed by the
Owens Valley Solar Array (OVSA). Emissions obtained by instruments in
satellites are discussed with emphasis to ultra-violet (UV) obtained by the
Transition Region And Coronal Explorer (TRACE), soft X-rays from the
Geostationary Operational Environmental Satellites (GOES) and X- and gamma-rays
from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The sub-THz
impulsive component had its closer temporal counterpart only in the higher
energy X- and gamma-rays ranges. The spatial positions of the centers of
emission at 212 GHz for the first flux enhancement were clearly displaced by
more than one arc-minute from positions at the following phases. The observed
sub-THz fluxes and burst source plasma parameters were found difficult to be
reconciled to a purely thermal emission component. We discuss possible
mechanisms to explain the double spectral components at microwaves and in the
THz ranges.Comment: Accepted version for publication in Solar Physic
Time-dependent Stochastic Modeling of Solar Active Region Energy
A time-dependent model for the energy of a flaring solar active region is
presented based on a stochastic jump-transition model (Wheatland and Glukhov
1998; Wheatland 2008; Wheatland 2009). The magnetic free energy of the model
active region varies in time due to a prescribed (deterministic) rate of energy
input and prescribed (random) flare jumps downwards in energy. The model has
been shown to reproduce observed flare statistics, for specific
time-independent choices for the energy input and flare transition rates.
However, many solar active regions exhibit time variation in flare
productivity, as exemplified by NOAA active region AR 11029 (Wheatland 2010).
In this case a time-dependent model is needed. Time variation is incorporated
for two cases: 1. a step change in the rates of flare jumps; and 2. a step
change in the rate of energy supply to the system. Analytic arguments are
presented describing the qualitative behavior of the system in the two cases.
In each case the system adjusts by shifting to a new stationary state over a
relaxation time which is estimated analytically. The new model retains
flare-like event statistics. In each case the frequency-energy distribution is
a power law for flare energies less than a time-dependent rollover set by the
largest energy the system is likely to attain at a given time. For Case 1, the
model exhibits a double exponential waiting-time distribution, corresponding to
flaring at a constant mean rate during two intervals (before and after the step
change), if the average energy of the system is large. For Case 2 the
waiting-time distribution is a simple exponential, again provided the average
energy of the system is large. Monte Carlo simulations of Case~1 are presented
which confirm the analytic estimates. The simulation results provide a
qualitative model for observed flare statistics in active region AR 11029.Comment: 25 pages, 9 figure
Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry -- Signatures of flow precession
NGC 7538 IRS1 is a high-mass (approx. 30 M_sun) protostar with a CO outflow,
an associated UCHII region, and a linear methanol maser structure, which might
trace a Keplerian-rotating circumstellar disk. The directions of the various
associated axes are misaligned with each other. We investigate the
near-infrared morphology of the source to clarify the relations among the
various axes. K'-band bispectrum speckle interferometry was performed at two
6-meter-class telescopes -- the BTA 6m telescope and the 6.5m MMT.
Complementary IRAC images from the Spitzer Space Telescope Archive were used to
relate the structures detected with the outflow at larger scales. High-dynamic
range images show fan-shaped outflow structure in which we detect 18 stars and
several blobs of diffuse emission. We interpret the misalignment of various
outflow axes in the context of a disk precession model, including numerical
hydrodynamic simulations of the molecular emission. The precession period is
approx. 280 years and its half-opening angle is 40 degrees. A possible
triggering mechanism is non-coplanar tidal interaction of an (undiscovered)
close companion with the circumbinary protostellar disk. Our observations
resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with
separation of 195 mas. We find indications for shock interaction between the
outflow activities in IRS1 and IRS2. Indications of outflow precession have
been discovered to date in a number of massive protostars, all with large
precession angles 20--45 degrees. This might explain the difference between the
outflow widths in low- and high-mass stars and add support to a common
collimation mechanism.Comment: 20 pages; 8 figures; Accepted by A&A on April 10, 2006; Image quality
reduced due to astro-ph file size limitations; Please download a version with
high-quality images from
http://www.mpifr-bonn.mpg.de/staff/tpreibis/ngc7538.pd
A 1.8 million year history of Amazon vegetation
During the Pleistocene, long-term trends in global climate were controlled by orbital cycles leading to high amplitude glacial-interglacial variability. The history of Amazonian vegetation during this period is largely unknown since no continuous record from the lowland basin extends significantly beyond the last glacial stage. Here we present a paleoenvironmental record spanning the last 1800 kyr based on palynological data, biome reconstructions, and biodiversity metrics from a marine sediment core that preserves a continuous archive of sediments from the Amazon River.
Tropical rainforests dominated the Amazonian lowlands during the last 1800 ka interchanging with surrounding warm-temperate rainforests and tropical seasonal forests. Between 1800 and 1000 ka, rainforest biomes were present in the Amazon drainage basin, along with extensive riparian wetland vegetation. Tropical rainforest expansion occurred during the relatively warm Marine Isotope Stages 33 and 31 (ca. 1110 to 1060 ka), followed by a contraction of both forests and wetlands until ca. 800 ka. Between 800 and 400 ka, low pollen concentration and low diversity of palynological assemblages renders difficult the interpretation of Amazonian vegetation. A strong synchronicity between vegetation changes and glacial-interglacial global climate cycles was established around 400 ka. After 400 ka, interglacial vegetation was dominated by lowland tropical rainforest in association with warmer temperatures and higher CO2. During cooler temperatures and lower CO2 of glacial stages, tropical seasonal forests expanded, presumably towards eastern Amazonia. While this study provides no evidence supporting a significant expansion of savanna or steppe vegetation within the Amazonian lowlands during glacial periods, there were changes in the rainforest composition in some parts of the basin towards a higher proportion of deciduous elements, pointing to less humid conditions and/or greater seasonality of precipitation. Nevertheless, rainforest persisted during both glacial and interglacial periods. These findings confirm the sensitivity of tropical lowland vegetation to changes in CO2, temperature, and moisture availability and the most suitable conditions for tropical rainforests occurred during the warmest stages of the Mid Pleistocene Transition and during the interglacial stages of the past 400 kyr
Low Velocity Ionized Winds from Regions Around Young O Stars
We have observed seven ultracompact HII regions in hydrogen recombination
lines in the millimeter band. Toward four of these regions, there is a high
velocity (full width to half maximum 60-80 km/s) component in the line
profiles. The high velocity gas accounts for 35-70% of the emission measure
within the beam. We compare these objects to an additional seven similar
sources we have found in the literature. The broad recombination line objects
(BRLOs) make up about 30% of all sources in complexes containing ultracompact
HII regions. Comparison of spectral line and continuum data implies that the
BRLOs coincide with sources with rising spectral indices, >=0.4 up to 100 GHz.
Both the number of BRLOs and their frequency of occurrence within HII region
complexes, when coupled with their small size and large internal motions, mean
that the apparent contradiction between the dynamical and population lifetimes
for BRLOs is even more severe than for ultracompact HII regions. We evaluate a
number of models for the origin of the broad recombination line emission. The
lifetime, morphology, and rising spectral index of the sources argue for photo-
evaporated disks as the cause for BRLOs. Existing models for such regions,
however, do not account for the large amounts of gas observed at supersonic
velocities.Comment: 36 pages, 8 figure
A near-infrared study of the NGC 7538 star forming region
We present sub-arcsecond (FWHM ~ 0".7), NIR JHKs-band images and a high
sensitivity radio continuum image at 1280 MHz, using SIRIUS on UH 88-inch
telescope and GMRT. The NIR survey covers an area of ~ 24 arcmin^2 with
10-sigma limiting mags of ~ 19.5, 18.4, and 17.3 in J, H, and Ks-band,
respectively. Our NIR images are deeper than any JHK surveys to date for the
larger area of NGC 7538 star forming region. We construct JHK CC and J-H/J and
H-K/K CM diagrams to identify YSOs and to estimate their masses. Based on these
CC and CM diagrams, we identified a rich population of YSOs (Class I and Class
II), associated with the NGC 7538 region. A large number of red sources (H-K >
2) have also been detected around NGC 7538. We argue that these red stars are
most probably PMS stars with intrinsic color excesses. Most of YSOs in NGC 7538
are arranged from the N-W toward S-E regions, forming a sequence in age: the
diffuse H II region (N-W, oldest: where most of the Class II and Class I
sources are detected); the compact IR core (center); and the regions with the
extensive IR reflection nebula and a cluster of red young stars (S-E and S). We
find that the slope of the KLF of NGC 7538 is lower than the typical values
reported for the young embedded clusters, although equally low values have also
been reported in the W3 Main star forming region. From the slope of the KLF and
the analysis by Megeath et al. (1996), we infer that the embedded stellar
population is comprised of YSOs with an age of ~ 1 Myr. Based on the comparison
between models of PMS stars with the observed CM diagram we find that the
stellar population in NGC 7538 is primarily composed of low mass PMS stars
similar to those observed in the W3 Main star forming region.Comment: 36 pages, 13 figures in JPEG format. Accepted for the publication in
ApJ. Report is also available at : http://www.tifr.res.in/~ojha/NGC7538.htm
Comparative functional analysis of aquaporins/glyceroporins in mammals and anurans
Maintenance of fluid homeostasis is critical to establishing and maintaining normal physiology. The landmark discovery of membrane water channels (aquaporins; AQPs) ushered in a new area in osmoregulatory biology that has drawn from and contributed to diverse branches of biology, from molecular biology and genomics to systems biology and evolution, and from microbial and plant biology to animal and translational physiology. As a result, the study of AQPs provides a unique and integrated backdrop for exploring the relationships between genes and genome systems, the regulation of gene expression, and the physiologic consequences of genetic variation. The wide species distribution of AQP family members and the evolutionary conservation of the family indicate that the control of membrane water flux is a critical biological process. AQP function and regulation is proving to be central to many of the pathways involved in individual physiologic systems in both mammals and anurans. In mammals, AQPs are essential to normal secretory and absorptive functions of the eye, lung, salivary gland, sweat glands, gastrointestinal tract, and kidney. In urinary, respiratory, and gastrointestinal systems, AQPs are required for proper urine concentration, fluid reabsorption, and glandular secretions. In anurans, AQPs are important in mediating physiologic responses to changes in the external environment, including those that occur during metamorphosis and adaptation from an aquatic to terrestrial environment and thermal acclimation in anticipation of freezing. Therefore, an understanding of AQP function and regulation is an important aspect of an integrated approach to basic biological research
Dense gas and the nature of the outflows
We present the results of the observations of the (J,K)=(1,1) and the
(J,K)=(2,2) inversion transitions of the NH3 molecule toward a large sample of
40 regions with molecular or optical outflows, using the 37 m radio telescope
of the Haystack Observatory. We detected NH3 emission in 27 of the observed
regions, which we mapped in 25 of them. Additionally, we searched for the
6{16}-5{23} H2O maser line toward six regions, detecting H2O maser emission in
two of them, HH265 and AFGL 5173. We estimate the physical parameters of the
regions mapped in NH3 and analyze for each particular region the distribution
of high density gas and its relationship with the presence of young stellar
objects. From the global analysis of our data we find that in general the
highest values of the line width are obtained for the regions with the highest
values of mass and kinetic temperature. We also found a correlation between the
nonthermal line width and the bolometric luminosity of the sources, and between
the mass of the core and the bolometric luminosity. We confirm with a larger
sample of regions the conclusion of Anglada et al. (1997) that the NH3 line
emission is more intense toward molecular outflow sources than toward sources
with optical outflow, suggesting a possible evolutionary scheme in which young
stellar objects associated with molecular outflows progressively lose their
neighboring high-density gas, weakening both the NH3 emission and the molecular
outflow in the process, and making optical jets more easily detectable as the
total amount of gas decreases.Comment: 27 pages, 37 figures. Accepted for publication in Astronomy and
Astrophysics. Abstract is abridge
- âŠ